The term “biomass” generally refers to the total amount of organisms that inhabit or exist in a given area. When such term is used with regard to plants, in particular, it refers to dry weight per unit area. Biomass units are quantified in terms of mass or energy. The expression “biomass” is synonymous with “the amount of living matter” or “the mass of an organism.” In the case of plant biomass, the term “standing crop” is occasionally used for “biomass.” Since plant biomass is generated by fixing atmospheric carbon dioxide with the use of solar energy, it can be regarded as so-called “carbon-neutral energy.” Accordingly, an increase of plant biomass is effective for global environmental preservation, the prevention of global warming, and mitigation of greenhouse gas emissions. Thus, technologies for increasing the production of plant biomass have been industrially significant.
Plants are cultivated for the purpose of using some tissues thereof (e.g., seeds, roots, leaves, or stems) or for the purpose of producing various materials, such as fats and oils. Examples of fats and oils produced from plants that have been heretofore known include soybean oil, sesame oil, olive oil, coconut oil, rice oil, cottonseed oil, sunflower oil, corn oil, safflower oil, palm oil, and rapeseed oil. Such fats and oils are extensively used for household and industrial applications. Also, fats and oils produced from plants are used as raw materials for biodiesel fuel or bioplastic, and the applicability thereof is increasing for alternative energy to petroleum.
In particular, an energy crop such as sugar cane can be used as a raw material for biofuel. Hence, the increased production of the total mass of a plant itself (the amount of plant biomass) is expected. Under such circumstances, improvement in productivity per unit of cultivation area is required in order to increase the production of plant biomass. It has been found that if the number of cultivated plants is assumed to be constant per unit of cultivation area, improvement in the amount of biomass per plant would be necessary.
However, it is thought that since many genes are involved in the amount of plant biomass (a so-called “kind of quantitative trait”), individual gene introduction or individual genetic modification cannot lead to an effective increase in production. Meanwhile, a great deal of difficulties are associated with introduction of many genes in a desired state into a plant. Such gene introduction is also problematic in that if successful introduction takes place, desirable traits cannot always be acquired.
Various gene introduction techniques are known as techniques for increasing the production of plant biomass, as disclosed in Patent Documents 1-7, for example. However, in the case of all thereof, attention is focused on the effects of increasing biomass production, while no technique for imparting salt stress resistance to a plant is disclosed.    Patent Document 1: JP Patent Publication (Kohyo) No. 2001-505410 A    Patent Document 2: JP Patent Publication (Kohyo) No. 2001-519659 A    Patent Document 3: JP Patent Publication (Kohyo) No. 2007-530063 A    Patent Document 4: JP Patent Publication (Kokai) No. 2005-130770 A    Patent Document 5: JP Patent Publication (Kohyo) No. 2000-515020 A    Patent Document 6: JP Patent Publication (Kohyo) No. 9-503389 (1997) A    Patent Document 7: JP Patent Publication (Kokai) No. 2005-52114 A